1. Field of the Invention
The present invention relates to an air/fuel ratio control system for an internal combustion engine, and more particularly a system in which an air/fuel ratio of mixture to be supplied to the engine is controlled basically in response to an output signal level of an oxygen concentration sensor.
2. Description of Background Information
Air/fuel ratio feedback control systems for an internal combustion engine are known as systems in which oxygen concentration in the exhaust gas of the engine is detected by an oxygen concentration sensor (referred to as O.sub.2 sensor hereinafter) and an air/fuel ratio of mixture to be supplied to the engine is feedback controlled in response to an output signal level of the O.sub.2 sensor for the purification of the exhaust gas and improvements of the fuel economy.
In this type of air/fuel ratio control system, a base value of the air/fuel ratio control is set in response to a plurality of engine parameters relating to the engine load, and the base value is corrected cyclically in response to the output signal level of the O.sub.2 sensor for elapse of a predetermined time period.
The feedback control of the air/fuel ratio in response to the output signal level of the O.sub.2 sensor is stopped under engine operational conditions such as a low load engine operation. During the stoppage of the feedback control of the air/fuel ratio, the air/fuel ratio of the mixture supplied to the engine is controlled to a rich air/fuel ratio value or a lean air/fuel ratio value. For this purpose, an opening degree of a solenoid valve provided for regulating the air/fuel ratio is controlled in accordance with a control value obtained by a multiplication between the previously set base value and an enrichment coefficient or a leaning coefficient. However, it is difficult to avoid the difference between the target air/fuel ratio and an actual air/fuel ratio of mixture because of various reasons such as the age-induced change in the detection characteristic of sensors for detecting the engine operational parameters, or the deterioration of the O.sub.2 sensor. Therefore, if, for example, the air/fuel ratio is controlled to the lean side to reduce the fuel consumption when the engine load is low, the air/fuel ratio of the mixture may not be precisely controlled to the desired value, causing an adverse effect on the driveability.